A method is know for producing an optical fiber by first drawing a perform into a fiber as it is melted in a resistance furnace, high-frequency furnace, with a CO.sub.2 laser, or with oxyhydrogen flame, and then coating the fiber with a resin composition before it contacts another solid object, followed by curing of the resin coating (see, for example, Japanese Patent Application (OPI) No. 100734/76, wherein the symbol OPI refers to an unexamined published Japanese Patent Application). This coating process is generally referred to as a tandem primary coating process, which, for industrial purposes, must be executed at high speed. To achieve this object, the following three requirements must be met.
First, the resin coating must be cured in a short period. It is known that a speed of at least several hundred meters per minute can be achieved by replacing the conventional electric furnace with an IR image furnace or UV irradiation furnace.
The second requirement originates from the fact that if the preform is drawn at a speed of 100 m/min or more at about 2000.degree. C., the fiber is supplied to a resin-filled coating apparatus before it is cooled to around room temperature, so that the resin composition is decomposed or cured in the coating apparatus. It is therefore necessary that some kind of cooling means be provided between the drawing furnace and the coating applicator. Japanese Patent Application (OPI) No. 10470/80 discloses one example of such cooling means through which a gaseous coolant (e.g., from liquid nitrogen) is made to flow to cool the fiber down to about room temperature. However, this idea of using a gaseous coolant has two problems: first, because of its small heat capacity, the gas has only a small cooling effect, and hence it must be used in a huge volume to cool the fiber to the desired temperature, and secondly, such gas typically contains a trace amount of moisture that reduces the strength of the fiber.
The third requirement is to minimize an uneven resin coating on the surface of optical fiber due to the relative slipping between the optical fiber and the resin composition when the fiber is coated at a high speed such as 100 m/min. or more.